Well, I think you're right about needing a fresh pair of eyes. And, as a pro writer you probably made less mistakes than most.
You paid low-to-below market rate to an ex-intern early stage career and I compared to a 20+ year proofreader in academic for ESL. Probably fishes to bicycles. I think you got what you needed which was a light touch confidence spellchecker.
A new to writing solo author, bursting with ten years of bildungsroman, who got Scriviner and has dumped two thousand disjoint paragraphs of prose into word...
I don't doubt that even a few hundred dollars may exceed revenue for a lot of self-published works. But I'd argue that someone needs to do a careful read-through of a book. You simply don't see mistakes after a while and most friends just aren't going to do a careful enough read through for free even if they're otherwise qualified.